Intraluminal, intracavity, intravascular, and intracardiac treatments and diagnosis of medical conditions utilizing minimally invasive procedures are effective tools in many areas of medical practice. These procedures are typically performed using imaging and treatment catheters that are inserted percutaneously into the body and into an accessible vessel of the vascular system at a site remote from the vessel or organ to be diagnosed and/or treated, such as the femoral artery. The catheter is then advanced through the vessels of the vascular system to the region of the body to be treated. The catheter may be equipped with an imaging device, typically an ultrasound imaging device, which is used to locate and diagnose a diseased portion of the body, such as a stenosed region of an artery. For example, U.S. Pat. No. 5,368,035, issued to Hamm et al., the disclosure of which is incorporated herein by reference, describes a catheter having an intravascular ultrasound imaging transducer.
An imaging transducer generally includes an imaging element configured to emit energy pulses. During operation, the imaging element is electrically excited, thus causing an energy pulse to be emitted. The pulse is directed to a surface where imaging is desired and reflected back to the transducer. Two desirable features of the emitted energy pulse are that the energy pulse be focused and steerable. One known approach known in the art to obtain these features is to utilize an array of imaging elements instead of just one element. FIG. 1 shows an array 10 of two imaging elements, A and B, side-by-side. As is known in the art, if both elements, A and B, are excited simultaneously, then the energy pulses are combined to form a beam that is parallel to the direction that the elements, A and B, are facing, so that the beam travels directly away from the array 10. However, if a linear timing excitation gradient (a time delay based on a coherence theory) is used across the array 10, the beam can be steered in the azimuthal direction. By sending a short acoustic pulse and receiving the echo at each azimuthal direction, the array 10 may scan a sector area and construct an image. The image resolution is primarily determined by the beam 20 width in the lateral direction and the acoustic pulse length in the axial direction.
To focus the beam, i.e., adjust the beam width, time delays for each element may also be utilized. At a certain spatial location, the acoustic pulses from all elements may be coherently enhanced when they are in phase. The phase of the pulse is determined by the distance from the element to the destination location. To focus the beam at a spatial point, appropriate time delays are applied to all of the elements, A and B. These compensating delays ensure that the arrival of the acoustic pulses from different elements, A and B, are in coincidence at the desired spatial location.
The array of imaging elements configured to enable a beam to be focused and steered is known in the art as a “phased array.” Though only two imaging elements, A and B, are shown in FIG. 1, a typical phased array may include as many as 256 elements. In the case of ultrasound imaging elements, each element, A and B, is generally small enough to be treated as an acoustic point source that generates a propagating wave with a spherical front. Collectively, the elements, A and B, form an acoustic field that can be enhanced when the elements, A and B, are in phase at a certain spatial location.
The elements, A and B, are typically rectangular and are typically evenly spaced across a flat plane. For ultrasound elements, each element has a pitch size equal to half a wavelength at the working ultrasound frequency. The pitch size is defined as the distance between two adjacent element, A and B, centers. With this typical configuration, when the beam 20 is steered and focused to a particular point F, the beams of the individual elements, A and B, are emitted at different angles, α and β, with respect to the flat plane. This will cause the beams of the individual elements, A and B, to have different amplitudes, which can undesirably result in a widened beam 20, even if an accurate time delay compensation is used. This is particularly so when the beam 20 is steered to the maximum azimuthal direction. Accordingly, an improved phased array imaging catheter would be desirable.